Method for desalinating saltwater using concentration difference energy

Abstract

Described herein are a method and system for desalinating saltwater using concentration difference energy. A “five stream” dialytic stack is described that can be used to desalinate saltwater at a relatively high recovery ratio. The dialytic stack may include, for example, one or more drive cells having a paired concentrate and a diluent-c chamber in ionic communication with a product chamber that is adjacent to an anion and a cation discharge chamber each filled with diluent-p. The drive cell applies a drive voltage across the product chamber, and when the drive voltage exceeds a desalination voltage of the product chamber, the saltwater in the product chamber is desalinated. The diluent-p may be at a lower ionic concentration than the diluent-c, which may be at a lower concentration than the concentrate. The relatively high concentrations of the concentrate and the diluent-c facilitate a relatively high recovery ratio, while the relatively low concentration of the diluent-p facilitates a relatively low desalination voltage. The dialytic stack may accept brine discharged from a first desalination plant and may use this brine as a source of the concentrate, diluent-c, or diluent-p. Alternatively, the dialytic stack may accept the brine as saltwater to be desalinated, and may then output desalinated brine back to the first desalination plant for further desalination. Processing the brine in the dialytic stack may decrease its volume, decreasing costs associated with treating or otherwise disposing of the brine.

1. A method for desalinating saltwater, comprising:
 generating a drive voltage by:
 flowing a concentrate solution through a concentrate chamber; and 
 flowing a diluent solution having an ionic concentration lower than an ionic concentration of the concentrate solution through a diluent chamber, the concentrate and diluent chambers forming a drive cell and sharing one of an anion and cation exchange membrane such that anions or cations flow from the concentrate chamber to the diluent chamber through the exchange membrane; 
 
 applying a sufficient voltage that comprises the drive voltage across a first product chamber through which flows the saltwater to be desalinated and that is in ionic communication with the drive cell, wherein the sufficient voltage equals or exceeds a desalination voltage of the first product chamber such that cations and anions respectively migrate from the first product chamber and into a first cation and first anion discharge chamber each bounding the first product chamber and respectively having flowing there through a first cation and first anion discharge solution; 
 wherein at least one of the first cation and first anion discharge solutions has an ionic concentration lower than the ionic concentration of the diluent solution. 
 
     
     
       2. A method as claimed in  claim 1 , wherein the first product chamber shares a first product chamber anion exchange membrane and a first product chamber cation exchange membrane with the first anion discharge chamber and the first cation discharge chamber, respectively, and wherein cations migrate from the first product chamber to the first cation discharge chamber through the first product chamber cation exchange membrane and anions migrate from the first product chamber to the first anion discharge chamber through the first product chamber anion exchange membrane. 
     
     
       3. A method as claimed in  claim 1 , wherein the diluent solution, the concentrate solution, and the first anion and first cation discharge solutions are all saltwater. 
     
     
       4. A method as claimed in  claim 1  wherein the ionic concentration of the first cation discharge solution and the ionic concentration of the first anion discharge solution are both lower than the ionic concentration of the diluent solution. 
     
     
       5. A method as claimed in  claim 4 , wherein the first anion and first cation discharge solutions are a common discharge solution. 
     
     
       6. A method as claimed in  claim 5  further comprising applying the sufficient voltage across a second product chamber through which also flows the saltwater being desalinated and that is in ionic communication with the drive cell, wherein the sufficient voltage equals or exceeds a sum of the desalination voltage of the first product chamber and a desalination voltage of the second product chamber such that cations and anions migrate from the second product chamber and into a second cation and second anion discharge chamber each bounding the second product chamber and having flowing there through a second cation and second anion discharge solution, respectively; and
 wherein the cation discharge chamber of one of the product chambers is also the anion discharge chamber of the other of the product chambers. 
 
     
     
       7. A method as claimed in  claim 6 , wherein the second anion discharge chamber contains the diluent solution. 
     
     
       8. A method as claimed in  claim 6 , wherein the ionic concentrations of one or both of the second anion and second cation discharge solutions are less than the ionic concentration of the diluent solution. 
     
     
       9. A method as claimed in  claim 6  further comprising periodically de-scaling the chambers by desalinating saltwater by:
 flowing the concentrate solution through the diluent chamber; 
 flowing the diluent solution through the concentrate chamber; 
 flowing the saltwater to be desalinated through the first cation and first anion discharge chambers; 
 flowing the first anion discharge solution through the second product chamber; and 
 flowing the first cation discharge solution through the first product chamber. 
 
     
     
       10. A method as claimed in  claim 1  further comprising when the concentrate solution is below a concentrate concentration threshold, heating the concentrate solution using waste heat from a process plant to facilitate evaporation to air, increasing the ionic concentration of the concentrate solution prior to flowing the concentration solution through the concentrate chamber by evaporating a portion of the concentrate solution to air. 
     
     
       11. A method as claimed in  claim 1 , wherein the first product chamber is at a higher pressure than the concentrate chamber, the diluent chamber, and the first anion and first cation discharge chambers. 
     
     
       12. A method as claimed in  claim 1  further comprising flowing an electrolyte through anode and cathode electrolyte chambers, the anode electrolyte chamber bounded on a first side by an anode stack end ion exchange membrane and bounded on another side by an anode, and the cathode electrolyte chamber bounded on a first side by a cathode stack end ion exchange membrane and bounded on another side by a cathode, the anode and cathode electrolyte chambers ionically communicative with the first product chamber via the anode and cathode stack end ion exchange membranes, respectively, and the anode and cathode electrically communicative with each other such that electrons flow from the anode to the cathode,
 wherein the anode and cathode electrolyte chambers are at a lower pressure than the concentrate chamber, the diluent chamber, the first anion and first cation discharge chambers, and the product chamber. 
 
     
     
       13. A method as claimed in  claim 1 , further comprising utilizing brine output from a desalination plant as a source of the concentrate solution and of the saltwater being desalinated, and wherein following exiting the first product chamber, the saltwater being desalinated is returned to the desalination plant for further desalination. 
     
     
       14. A method as claimed in  claim 13 , wherein the brine output has an ionic concentration between that of the concentrate solution and the diluent solution. 
     
     
       15. A method as claimed in  claim 13 , wherein the brine output is used as a source of any one or more of the diluent solution and the first anion and first cation discharge solutions.